Method and apparatus for descaling an evaporator effect



A. N. CHIRICO June 28, 1966 METHOD AND APPARATUS FOR DESCALING AN EVAPORATOR EFFECT Filed. June 1, 1964 M030: huacomm M m 4 2 j y ,2 6 $1; a x26] Qwuu /K QN w Z m @u wP wZuDZOU -QQN A Ydfl a? $1 United States Patent 3,258,060 METHOD AND APPARATUS FOR DESCALING AN EVAPURATOR EFFECT Anthony N. Chirico, Naperville, Ill., assignor to Chicago Bridge and iron Company, Oak Brook, IlL, a corporation of Illinois Filed June 1, 1964, Ser. No. 371,546 7 Claims. (Cl. 159-20) This invention relates to multiple effect evaporation of scale-forming liquors. More particularly, the invention relates to a method and apparatus for washing an evaporator effect to remove scale while evaporation is continued in the remainder of the evaporator.

Multiple effect evaporation of various liquors frequently is accompanied by the formation of scale on the liquor side of one or more evaporator effects. The scale reduces the heat transfer coefficient and consequently the capacity of the evaporator, making it necessary to remove the scale periodically. The problem of scaling is especially acute when the scale deposits are insoluble in water. For example, severe scaling problems are. encountered in the evaporation of waste liquor from the manufacture of sulfite pulp and in the evaporaion of saline liquors such as seawater brines. The problems are accentuated when the evaporator is shut down for cleaning and the scale is dehydrated, causing the scale to become hard and more resistant to removal.

Various methods have been employed in the past to remove scale. In many cases, the entire evaporator is shut down, with consequent loss of production. The scale is removed by boiling out the evaporator with water, alkali and/ or acid, or by drilling out the evaporator tubes with a scale-removing tool. Even when operating for optimum periods of time between shutdowns for scale removal, it is necessary to contend with an undesirable average amount of scale on the heat transfer surfaces. Other methods for removing scale have been employed, which do not require shutdown of the evaporator. These methods involve bypassing the liquor and the vapor around the evaporator effect being washed. Such methods impose additional requirements for relatively expensive vapor piping. The additional vapor piping results in increased air leakage into the system and increases the venting requirements. The methods also involve blanking off sections of the vapor piping, which retards operation and increases the labor requirements.

The present invention provides a new and improved method and apparatus for evaporating a scale-forming liquor which overcome prior problems and provide substantial advantages over the prior methods and apparatus. In the invention, vapor condensate resulting from evaporation is employed to wash the liquor side of one or more evaporator effects to remove insoluble scale formed thereon while the remainder of the evaporator is on stream or in production. In the preferred embodiments of the invention, liquor is bypassed around an evaporator effect to be washed while vapor is conducted to the vapor side of the effect. Vapor condensate is conducted to the liquor side of the effect for washing the effect. In a more specific preferred method as described and claimed herein, the vapor condensate is conducted to the liquor side from the vapor side of an evaporator effect, and the condensate is boiled in contact with the liquor side. In another specific me-thod of washing the liquor side of an evaporator effect with vapor condensate, described and claimed together with apparatus adapted for performing the method in my copending application Serial No. 367,665, filed May 15, 1964, the effect is washed with vapor condensate at a relatively low temperature.

The present invention provides improvements in multiple effect evaporator construction adapted for performing the new method and providing a number of advantages. A multiple effect evaporator including vapor transfer conduits interconnecting respective liquor and vapor sides (i.e. steam sides) of successive evaporator effects, and liquor transfer conduits interconnecting respective liquor sides of the evaporator effects in a given sequence, is provided with the combination of a liquor bypass conduit interconnecting respective liquor sides of two alternate (i.e. nonadjacent) evaporator effects in the sequence for bypassing liquor around .the intervening evaporator effect, and a condensate transfer conduit interconnecting the vapor side (i.e. steam side) of one of the evaporator effects and the liquor side of the intervening effect for conducting vapor condensate from the former side to the latter side for washing the latter side.

Employing the method and apparatus of the invention, the evaporator may be operated uninterruptedly while an effect is washed continuously. Scale accumulation is kept at a minimum, and the heat transfer coefficient is maintained near the maximum value. At the same time, the scale is prevented from becoming dehydrated.

The invention is especially advantageous for use in the evaporation of liquors that form insoluble scale and result in the formation of acidic vapors and vapor condensates. Thus, for example, spent ammonia base sulfite liquor forms an insoluble scale upon evaporation. The vapor condensate obtained in the evaporation process has a pH of about 1.5-2, and it has been found to be very effective in removing the scale which is deposited from the liquor.

The new method and apparatus provide high thermal efficiencies. The vapor produced in boiling the condensate employed for washing is conducted to the next succeeding effect to supply heat for evaporation therein, in the same manner as vapor is conducted to the latter effect when the vapor is produced by evaporation of the liquor being concentrated. The wash liquid resulting from washing may be conducted to another effect to utilize its heat content. In cases where it is unnecessary to wash the evaporator continuously, the entire evaporator may be place-d on stream when not being washed, with resulting increased capacity and economy.

The invention does away with the need for bypassing vapor (i.e. steam) around the effect being washed. By minimizing the vapor piping, air leakage into the system and venting requirements are minimized. Vapor bypass piping is replaced by less expensive condensate piping. The operations of placing an effect on a washing cycle and transferring the washing cycle from one effect to another are easily and rapidly accomplished, and they may be performed and controlled automatically.

The foregoing and other improvements and advantages of the invention will be apparent on reference to the specification and to the attached drawing illustrating the invention. The drawing illustrates diagrammatically a quadruple effect evaporator in operation.

In the illustrative embodiments of the method and apparatus, evaporation takes place in concurrent flow, wherein the liquor flows in forward feed from the first effect to the fourth effect. The invention also is applicable to counterflow evaporation, wherein backward feed is employed, and to mixed flow evaporation, wherein both forward and backward feed is employed.

The evaporator includes successive first through fourth effects 10, ll, 12 and 13, respetively. Each effect constitutes a conventional natural circulation evaporator effect of the long tube vertical falling film type having a single tube pass. The invention also is applicable to evaporation in forced circulation evaporator effects and to evaporation in any other type of evaporator effect including, without limitation thereto, the vertical climbing film type. The evaporator effects may include provision for two or more tube passes.

Each of the illustrative effects -13 includes a vertically arranged tubular or tube-in-shell heater, designated 10a to 13a for the respective effects. The heater includes an inner tube bundle 14 and an outer shell 15. The interior of the tubes consitutes the liquor or tube side of the heater, and the exterior of the tubes and the interior of the shell consitute the vapor or shell side (i.e. steam side) of the heater and also of the evaporator effect. Each effect includes a liquor box or inlet chamber surmounting the heater, respectively designated 10b to 13b, and a vapor body or disengaging vessel 'at the lower end of the heater, respectively designated 100 to 130. The interiors of the liquor box and the vapor body are in communication with the interior of the heater tubes, and together they constitute the liquor side of the effect.

The heater of each effect is supplied with steam or other condensable heating medium on the vapor side (i.e. steam side). The liquor box of each effect is supplied with liquor to be evaporated, and the liquor falls through the tubes in the heater and is heated and vaporized therein. The vapor-liquid mixture is discharged into the vapor body, where vapor separates from concentrated liquor. The vapor formed in each effect on the liquor side is conducted from the vapor body to the vapor side (i.e. steam side) of the next succeeding effect, in the heater thereof, where it is employed as a heating medium. When the entire evaporator is on stream, the concentrated liquor produced in each effect on the liquor side is conducted from the vapor body to the liquor side of the next succeeding effect, being supplied to the liquor box thereof, for additional evaporation.

Feed liquor is supplied to the liquor box 1% of the first effect 10 through a liquor supply conduit or pipe 10d communicating with the interior of the liquor box. Similarly, liquor is supplied to the liquor boxes of the successive effects through respective liquor supply conduits 11d13d. Liquor discharge conduits 10e-13e are connected to the respective vapor bodies 10c-13c, at their bottoms and in communication with their lower interior portions. Liquor transfer conduits 10f12f are connected to the respective discharge conduits 10e-12e for the first, second and third effects and to the respective liquor supply conduits 11d-13d for the second, third and fourth effects, to thereby interconnect respective liquor sides of successive effects. The product is removed through the discharge conduit 13c of the fourth effect.

Steam is employed as the condensable heating medium in the first effect 10. The steam is supplied to the heater 10a of the effect through a vapor supply conduit 10g communicating with the interior of the heater shell 15. Similarly, condensable vapor is supplied to the vapor sides (i.e. steam sides) of the successive effects 11-13 through vapor supply conduits 11g-13g connected to the respective heaters Ila-13a thereof. Vapor transfer conduits 10h-12h are connected to the respective vapor bodies 10c-12c of the first three effects in communication with the vapor spaces therein. The vapor transfer conduits are connected to the vapor supply conduits 11g-13g on the respective successive heaters to thereby interconnect respective liquor and vapor sides of successive evaporator effects. A like vapor transfer conduit 13h on the vapor body 13c of the fourth effect may be connected to the heater of an additional effect in like manner or to a condenser, not shown.

A vapor condensate transfer or discharge conduit 10k is connected to the heater 10a of the first effect 10 in communication with the vapor side (i.e. steam side) thereof. The clean steam condensate formed in the first effect is conducted to heat exchanger 75 for preheating the feed, after which it may be returned to the steam boiler. Similarly, condensate transfer conduits 11k-13k are connected to the respective heaters Ila-13a of the successive effects. The condensate transfer conduit 11k of the second effect 11 also is connected to the third effect heater 12a to thereby interconnect the respective vapor sides of the successive effects. Similarly, the vapor sides of the third and fourth effects 12 and 13 are interconnected by the condensate transfer conduit 12k connected to the respective heaters 12a and 13a thereof. The condensate transfer conduits 11k and 12k serve to transfer or conduct vapor condensate from one effect to the next effect to utilize part of the heat content of the condensate for evaporation in the succeeding effect. Similarly, vapor condensate may be transferred from the fourth effect 13 to an additional effect, not shown, or its heat content may be utilized in other ways, such as to preheat the feed.

Evaportion may be conducted in the apparatus as so far described in a conventional manner. Liquor is conducted through the evaporator in forward feed, from the first effect 10 to the fourth effect 13. Steam is supplied to the first effect, and vapor is conducted from each effect to the next effect. Vapor condensate formed in the first effect is withdrawn from the evaporator. Vapor condensate formed in the second and third effects is conducted to the respective successive effects.

In accordance with the invention, additional conduits are provided for washing an evaporator effect when scale forms on the liquor side thereof. In the illustrative embodiment, a liquor bypass conduit 16 is connected to the liquor transfer conduits 10 and 11] of the first and second effects 10 and 11, respectively. Valves 17 and 18 are provided in-the transfer conduits, and valves 19 and 20 are provided in the bypass conduit, for directing the flow of liquor, or other suitable means may be provided. The bypass conduit 16 interconnects the respective liquor sides of the alternate first and third effects 10 and 12 for bypassing liquor around the intervening second effect 11. Similarly, bypass conduits are provided for bypassing other evaporator effects as described hereinafter.

A branch condensate transfer conduit 21 is connected to the main condensate transfer conduit 12k on the third effect 12. The branch condensate transfer conduit also is connected to the liquor supply conduit 11d on the second effect 11. A valve 22 is provided in the main condensate transfer conduit, and valves 23 and 24 are provided in the branch conduit, for directing the flow of condensate. The branch conduit interconnects the vapor side (i.e. steam side) of the third effect and the liquor side of the second effect for conducting vapor condensate from the former side to the latter side. Condensate is transferred in this manner by means of a pump 25 connected to the branch conduit. The condensate is conducted to the second effect in backward flow from the next succeeding third effect for washing the liquor side of the second effect. The illustrative method and apparatus provide desirable thermal efficiency. Alternatively, vapor condensate for washing may be obtained from other effects and transferred in backward or forward flow. Thus, in forward flow, vapor condensate can be obtained from effect 12, passed through valve 24 to conduit 40 and through valve 41 to conduit 13d to the liquor side of effect 13 to wash the same. While washing effect 13, liquor from effect 12 can be passed by conduit 12c, through valve 44 (with valve 42 closed) and conduit 43 to conduit 13a and out therefrom as product liquor thus bypassing effect 13.

To wash effect 12, liquor from effect 11 is conveyed by conduit 11e (valves 32 and 18 closed) through valves 31 and 56 (valve 54 closed) and conduit 57 (valve 42 closed) to conduit 13d and thus to effect 13 thereby bypassing effect 12. Washing of effect 12 can then be effected by taking vapor condensate from effect 13 through conduit 13k to conduit 50, through open valve 51 and conduits 52 and 53 to the liquor side of effect 12.

Effect 10 is readily washed by first closing valve 70, opening valve 71 and conveying feed liquor by conduit 72 to conduit 11d (valve 17 closed) and thus to the liquor side of effect 11. This course thus bypasses liquor around effect so it may be cleaned. Since effect 11 will not be receiving as much vapor through conduits 10h and 11g to keep it heated, steam can be fed by conduit 80 with valve 81 open to the tube side of effect 11 and the condensate removed by conduit 82 through valve 83. To wash effect 10, condensate transfer pump 25 can pump condensate from the tube side of effect 12, with valve 23 closed, to conduit 90, through valve 91 to conduit 92, and through valve 93 to the liquor side of effect 10 to wash the same.

A wash transfer conduit 30 is connected to the liquor transfer conduit 11e from the second effect. The wash condensate transfer conduit also is connected to the main condensate transfer conduit 12k interconnecting the sec- 0nd and third effects. A valve 31 is provided in the wash transfer conduit, for directing the flow of wash liquid. The wash transfer conduit interconnects the liquor side of the second effect 11 and the vapor side of the fourth effect 13 for conducting wash liquid from the former side to the latter side. Part of the heat content of the wash liquid may be utilized for evaporation in the fourth effect by conducting the liquid thereto. Similar conduits may be connected to other effects for transferring the wash liquid. Thus, wash liquid from effect 12 can be sent by conduits 12a, 55 and 12k to the vapor side of effect 13 with valves 42 and 54 closed and valve 58 open. Alternatively, the wash liquid may be withdrawn from each evaporator. Thus, a drain valve 32 is provided in the liquor discharge conduit 1112 on the second effect, and conduit 60 and valve 61 on third effect, for this purpose.

The illustrative method and evaporator are especially adapted for concentrating spent ammonia base sulfite liquor which has been partly concentrated and preheated to a temperature that is in the vicinity of its boiling point in the first effect 10. For example, the feed liquor may enter the first effect liquor box 10b at a temperature on the order of 255 F. and with a total solids concentration of about 19% by weight. Steam is supplied to the first effect through the vapor supply conduit 10g at a pressure of about 50 pounds per square inch gauge. The temperature of the steam and the vapor condensate is about 300 F. The boiling point of the liquor in the first effect may be on the order of 285 F. The temperature of the process liquor in the fourth effect 13 is about 230 F. while the vapor temperature is about 222 F. A concentrated liquor product is obtained and removed from the fourth effect through the liquor discharge conduit 13a and with a total solids concentration of about 60% as is desirable for charging to a furnace in the conventional manner for disposing of the waste material. Evaporation at such high temperatures and concentrations causes large quantities of insoluble scale to deposit on the liquor sides of the evaporator effects. Consequently, it is necessary that the effects be washed frequently. For example, daily washing of each effect is required when evaporation takes place at a rate on the order of 80,000 pounds per hour.

The vapor produced in the first effect 10 separates from the concentrated liquor in the vapor body 100. The vapor is conducted to the vapor side of the second effect 11 through the vapor transfer conduit 10h. The concentrated liquor is conducted from the vapor body 100 to the liquor side of the third effect 12, through the liquor transfer conduit 10f and the liquor bypass conduit 16. Vapor condensate from the third effect 12 is conducted to the liquor side of the second effect 11 through the branch condensate transfer conduit 21. The condensate is substantially at its boiling point at the pressure existing on the liquor side of the second effect. The condensate is caused to boil vigorously in the second effect by the heat content of the vapor supplied to the vapor side thereof from the first effect. Scale is effectively removed from the liquor side of the second effect.

The boiling wash condensate collects in the vapor body of the second effect. The vapors which separate in the vapor body are conducted to the vapor side of the third effect through the vapor transfer conduit 11h. The vapor condensate formed in the second effect heater 11a is conducted to the vapor side of the third effect through the condensate transfer conduit 11k. The wash liquid which separates in the vapor body 11c is conducted to the vapor side of the fourth effect 13 through the wash transfer conduit 30. Alternatively, the wash liquid may be removed from the evaporator through the drain valve 32.

The vapor and the vapor condensate produced in the washing cycle in the second effect 11 are employed for concentrating the liquor from the first effect 10 in the third effect 12. The vapor separated in the vapor body 12c of the third effect is conducted to the vapor side of the fourth effect 13 through the vapor transfer conduit 12h. The concentrated liquor is conducted from the vapor body of the third effect to the liquor side of the fourth effect through the liquor transfer conduit 12 While the second effect 11 is in its washing cycle, the vapor condensate from the third effect is conducted to the second effect, as previously described. At the same time, wash liquid is conducted to the vapor side of the fourth effect from the vapor body of the second effect to provide a heating medium in the fourth effect in addition to the vapor heating medium from the third effect.

The liquor is further concentrated in the fourth effect 13. Vapor is removed from the vapor body 13c thereof and conducted through the vapor transfer conduit 13h either to an additional effect or to a condenser. Vapor condensate is conducted from the fourth effect through the condensate conduit 13k to an additional effect or to a heat exchanger. Product liquor is withdrawn from the fourth effect through the liquor discharge conduit 136.

After the second effect 11 has been washed, it is placed on stream once more. For this purpose, the valves 19 and 20 connected to the liquor bypass conduit 16, the valves 23 and 24 connected to the branch condensate conduit 21, and the valve 31 connected to the wash transfer conduit 30 are closed. The valves 17 and 18 connected to the respective first 'and second effect liquor transfer conduits 10 and 11f are opened for conducting liquor from the first effect to the second effect and from the second effect to the third effect. The valve 22 connected to the main condensate conduit 12k is opened for conducting vapor' condensate from the third effect to the fourth effect. The several vapor transfer conduits 10h-13h remain open continuously during operation of the evaporator. The remaining effects may be washed in like manner employing accessory liquor bypass and branch condensate transfer conduits respectively similar to the bypass conduit 16 and branch conduit 21. Wash transfer conduits similar to the conduit 30 also may be provided.

The invention thus provides a new and improved meth- 0d and apparatus for continuously evaporating an insoluble scale-forming liquor in a multiple effect evaporator while washing one or more effects continuously or intermittently. Scale accumulation is minimized, optimum heat transfer coefficients are achieved, and high thermal efiiciencies are obtained. The apparatus includes only the vapor piping necessary for evaporation, and less expensive condensate piping is provided for washing purposes. The washing operations are conducted easily and rapidly.

While preferred embodiments of the method and apparatus have been described and illustrated, it will be apparent that various changes and modifications may be made therein within the spirit and scope of the invention. It is intended that such changes and modifications be included within the scope of the appended claims.

I claim:

1. In a method of evaporating a scale-forming liquor in a multiple effect evaporator wherein a vapor condensate is produced, the improvement which comprises bypassing said liquid around an evaporator effect I to another effect II while continuing to conduct vapor from an effect other than I and at a temperature higher than the latter to the vapor side of effect I and maintaining the remainder of the evaporator on stream, conducting said vapor condensate from the vapor side of said evaporator effect II, said vapor condensate being at a lower temperature than the vapor in the vapor side of effect I, to the liquor side of evaporator effect I, and boiling said vapor condensate in contact with said liquor side of evaporator effect I to remove scale formed thereon.

2. The method of claim 1 in which evaporator effect I is other than the first and last effects with respect to liquor How. A

3. In a method of evaporating a scale-forming liquor in a multiple effect evaporator wherein a vapor condensate is produced, the improvement which comprises bypassing said liquor around an evaporator effect I while conducting vapor to the vapor side thereof and maintaining the remainder of the evaporator on stream, conducting said vapor condensate from the vapor side of the next succeeding evaporator effect II to the liquorside of said first-named effect I, boiling said vapor condensate in contact with said liquor side of effect I to remove scale formed thereon, and conducting vapor from said liquor side of effect I to the vapor side of said succeeding effect If.

4. In a multiple effect evaporator including vapor transfer conduits interconnecting respective liquor and vapor sides of successive evaporator effects, and liquor transfer conduits interconnecting respective liquor sides of said evaporator effects in series, the combination of a liquor bypass conduit interconnecting respective liquor sides of two nonadjacent evaporator effects in said sequence for bypassing liquor around an intervening evaporator effect, and a condensate transfer conduit interconmeeting the vapor side of one of said nonadjacent evaporator effects and the liquor side of said intervening effect for conducting vapor condensate from the vapor side of the nonadjacent effect to the liquor side of the intervening effect for washing the liquor side thereof with respect to liquor flow.

5. An evaporator as defined in claim 4 wherein said one evaporator effect comprises the nonadjacent evaporator effect which succeeds said intervening evaporator effect.

6. In a multiple effect evaporator including vapor transfer conduits interconnecting respective liquor and vapor sides of successive evaporator effects in series, condensate transfer conduits interconnecting respective vapor sides of successive evaporator effects, and liquor transfer conduits interconnecting respective liquor sides of said evaporator effects in series, the combination of a liquor bypass conduit interconnecting respective liquor sides of two nonadjacent evaporator effects in said sequence for bypassing liquor around an intervening evaporator effect, a condensate transfer conduit interconnecting the vapor side of one of said nonadjacent effects and the liquor side of said intervening effect for conducting vapor condensate from the nonadjacent effect to the liquor side of the intervening effect for washing the liquor side thereof, said one nonadjacent effect succeeding said intervening effect, and a wash transfer conduit interconnecting the liquor side of said intervening effect and the vapor side of an evaporator effect succeeding said one nonadjacent effect for conducting wash liquor from the liquor side of the intervening effect to the vapor side of the evaporator effect succeeding said one nonadjacent effect.

7. In a multiple effect falling film evaporator, having forward or concurrent flow, including vapor transfer conduits interconnecting respective liquor and vapor sides of successive effects in series, condensate transfer conduits interconnecting respective vapor sides of successive evaporator effects, and liquor transfer conduits interconnecting respective liquor sides of said evaporator effects in series, the combination of a liquor bypass conduit interconnecting respective liquor sides of two nonadjacent evaporator effects in said sequence with respect to liquor flow for bypassing liquor around an intervening evaporator effect, a condensate transfer conduit interconmeeting the vapor side of one of said nonadjacent effects and the liquor side of said intervening effect for conducting vapor condensate from said one nonadjacent effect to the liquor side of the intervening effect for washing the liquor side thereof, said one nonadjacent effect succeeding said intervening effect with respect to liquor flow, and a wash transfer conduit interconnecting the liquor side of said intervening effect and the vapor side of an evaporator effect succeeding said one nonadjacent effect for conducting wash liquid from the liquor side of the intervening effect to the vapor side of the evaporator effect succeeding said one nonadjacent effect.

References Cited by the Examiner UNITED STATES PATENTS 2,647,570 4/1953 Lock-man.

2,734,565 2/1956 Lockrnan l5920 2,781,089 2/1957 Mair et a1. 15920 2,796,120 6/1957 Lockman et a1 a 159-20 X NORMAN YUDKOFF, Primary Examiner.

J. SOFER, Assistant Examiner. 

1. IN A METHOD OF EVAPORATING A SCALE-FORMING LIQUOR IN A MULTIPLE EFFECT EVAPORATOR WHEREIN A VAPOR CONDENSATE IS PRODUCED, THE IMPROVEMENT WHICH COMPRISES BYPASSING SAID LIQUID AROUND AN EVAPORATOR EFFECT I TO ANOTHER EFFECT II WHILE CONTINUING TO CONDUCT VAPOR FROM AN EFFECT OTHER THAN I AND AT A TEMPERATURE HIGHER THAN THE LATTER TO THE VAPOR SIDE OF EFFECT I AND MAINTAINING THE REMAINDER OF THE EVAPORATOR ON STREAM, CONDUCTING SAID VAPOR CONDENSATE FROM THE VAPOR SIDE OF SAID EVAPORATOR EFFECT II, SAID VAPOR CONDENSATE BEING AT A LOWER TEMPERATURE THAN THE VAPOR IN THE VAPOR SIDE OF EFFECT I, TO THE LIQUOR SIDE OF EVAPORATOR EFFECT I, AND BOILING SAID VAPOR CONDENSATE IN CONTACT WITH SAID LIQUOR SIDE OF EVAPORATOR EFFECT I TO REMOVE SCALE FORMED THEREON. 